Of course, this isn't the end of the story. Far from being pointless, Evolution comes with a pre-defined view that is follow-up flag organization on steroids. Not only does it display each message's follow-up flag, can you also sort by flag and see the due date, by which you can also sort (and see the flag).
To organize a mailbox by follow-up status easily in Evolution:
- Select View | Current View | By Follow Up Flag from the menu.
Should you ever want to go back to the "normal" message view, choose View | Current View | Messages.

